1. Field of the Invention
The present invention relates to an image forming apparatus having a fixing unit which fixes a toner image onto a transfer medium in accordance with a heat roller scheme that uses a heat roller and press roller.
2. Description of the Prior Art
Conventionally, in an image forming apparatus such as a copying machine, a printer, or a facsimile apparatus, a fixing unit which heats and fixes a recording medium (to be referred to as a transfer medium hereinafter) for carrying toner made of a heat-melting resin or the like often employs a heat roller scheme.
The heat roller type fixing unit is formed of a heat roller having a heat generating source such as a halogen heater in it, and a press roller pressed against it. A recording medium (to be referred to as a transfer medium hereinafter) such as a paper sheet is passed through a nip portion as a press contact portion of the pair of rollers that are being rotated, so that toner which is made of a heat-melting resin or the like and carried by the transfer medium is heated and fixed.
Each of the heat roller and press roller undergoes heat and pressure repeatedly. Then, the tensile strength and/or modulus of impact resilience decreases due to heat deterioration of the resilient layer as one constituent element of each roller, and the surface hardness of each roller changes, to break the roller. Therefore, the roller must sometimes be changed before it reaches a predetermined roller service life.
The heat roller type fixing unit conveys a transfer medium while fixing a toner image on it by rotation of the heat roller. Accordingly, the transfer medium may sometimes twine around the heat roller due to fusion of the toner. To prepare for such a case, the fixing unit employs, e.g., a method with which a nip portion forms an upward projection (an upper heat roller 41 arcuately deforms at a hip portion T as shown in FIG. 1) so that the separation performance of the transfer medium improves, or a separation pawl method with which a pawl for transfer medium separation is forcibly pressed against the heat roller.
FIG. 1 is a view showing how the transfer medium is conveyed at the nip portion.
As shown in FIG. 1, the heat roller 41 and a press roller 42 are formed of cylindrical metal cores 413 and 423, annular resilient layers 412 and 413 made of heat resistant rubber baked on the metal cores 413 and 423, and release layers 411 and 412 that cover the resilient layers 412 and 422, respectively. The press contact portion of the two rollers 41 and 42 forms the nip portion T.
According to the method of separating a transfer medium P by employing the nip portion T that forms an upward projection, the surface hardness of the heat roller 41 is decreased to be smaller than that of the press roller 42 (an angle xcex8 of the heat roller 41 and transfer medium P during separation increases). Thus, the heat roller 41 clamps and conveys the transfer medium P while being deformed by the press roller 42. The impact resilience force (restoration force) of the heat roller 41 at the moment the transfer medium P is released from the nip portion T works in the direction of an arrow R, and the transfer medium P is separated.
The heat roller and press roller are usually exchanged during periodic inspection or the like when a predetermined use (copy) amount (100,000 copies in this embodiment) is reached. The hardnesses of the heat resistant rubber members 412 and 422 as the resilient layers (to be also referred to as resilient bodies hereinafter) change before the periodic inspection due to heat deterioration caused by repeated thermal stress or the like, and the moduli of the impact resilience of the resilient layers 412 and 422 sometimes decrease. In this case, since the impact resilience is insufficient, the heat roller 41 loses its stable separation performance, and causes defective separation.
According to the separation pawl method, the pawl may damage the heat roller, and vertical lines may accordingly be formed in the image, leading to a degradation in image quality.
The present invention has been made to solve the problems described above, and has as its object to provide an image forming apparatus having a heat roller type fixing unit with an improved durability.
It is another object of the present invention to provide an image forming apparatus having a heat roller type fixing unit that exhibits a stable separation performance.
In order to achieve the above objects, according to the main aspect of the present invention, there is provided an image forming apparatus comprising a fixing unit which includes a heat section having a resilient body and a release layer that covers the resilient body, and a press section having a resilient body and a release layer that covers the resilient body and pressed by the heat section, and which heats and fixes a transfer medium at a nip portion as a press contact portion of the two sections, wherein
at least one of the following three conditions is satisfied:
(a) a tensile strength of the resilient body of the heat section upon a lapse of an arbitrary use time is higher than an initial tensile strength thereof or is not lower than the initial tensile strength by not less than 0.5 MPa,
(b) a relationship (H1xe2x88x92H0)xe2x88x92(h1xe2x88x92h0)xe2x89xa60 is always satisfied where H0 is the initial surface hardness of the heat section, h0 is the initial surface hardness of the press section, and H1 and h1 are the surface hardnesses of the heat section and the press section, respectively, upon a lapse of the arbitrary use time, and
(c) a modulus of impact resilience of the resilient layer of the heat section upon a lapse of the arbitrary use time is higher than a modulus of initial impact resilience thereof before start of use, or is not lower than the modulus of initial impact resilience by not less than 10%.
The image forming apparatus according to the main aspect has the following subsidiary aspects.
(1) The initial tensile strength of the heat section is not less than 0.6 MPa.
(2) A temperature of the press section is set within such a temperature range that the tensile strength of the resilient body thereof does not decrease.
(3) SP less than A and SP less than B are always satisfied where SP is the surface pressure at the nip portion, A is the tensile strength of the resilient body of the heat section, and B is the tensile strength of the resilient body of the press section.
(4) A relationship between the initial surface hardness H0 of the heat section and the initial surface hardness h0 of the press section satisfies H0 less than h0.
(5) 40%xe2x89xa6E0xe2x89xa660% is satisfied where E0 is the modulus of initial impact resilience of the resilient body of the heat section.
(6) The resilient body of the heat section uses such a member that the modulus of impact resilience thereof is always larger than that of the resilient layer of the press section.
(7) The heat section and the press section comprise a heat roller and press roller, respectively, and a diameter of the heat roller is larger than that of the press roller.
As is apparent from the above aspects, according to the present invention, heat resistant rubber, whose tensile strength does not decrease, or decreases only by a small amount, by the heat deterioration of the heat section and press section is used. Thus, surface breaking or separation from the metal core is eliminated, and the durability is improved.
Also, according to the present invention, a resilient member the modulus of impact resilience of which does not change easily by heat deterioration is used for each of the heat section and press section without changing the arrangement of the fixing unit. Thus, the separation performance of the transfer medium from the heat section can be improved.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.